From Homogeneous to Heterogenized Molecular Catalysts for H2 Production by Formic Acid Dehydrogenation: Mechanistic Aspects, Role of Additives, and Co-Catalysts

Abstract

H2 production via dehydrogenation of formic acid (HCOOH, FA), sodium formate (HCOONa, SF), or their mixtures, at near-ambient conditions, T < 100 °C, P = 1 bar, is intensively pursued, in the context of the most economically and environmentally eligible technologies. Herein we discuss molecular catalysts (ML), consisting of a metal center (M, e.g., Ru, Ir, Fe, Co) and an appropriate ligand (L), which exemplify highly efficient Turnover Numbers (TONs) and Turnover Frequencies (TOFs) in H2 production from FA/SF. Typically, many of these ML catalysts require the presence of a cofactor that promotes their optimal cycling. Thus, we distinguish the concept of such cofactors in additives vs. co-catalysts: When used at high concentrations, that is stoichiometric amounts vs. the substrate (HCOONa, SF), the cofactors are sacrificial additives. In contrast, co-catalysts are used at much lower concentrations, that is at stoichiometric amount vs. the catalyst. The first part of the present review article discusses the mechanistic key steps and key controversies in the literature, taking into account theoretical modeling data. Then, in the second part, the role of additives and co-catalysts as well as the role of the solvent and the eventual inhibitory role of H2O are discussed in connection to the main mechanistic steps. For completeness, photons used as activators of ML catalysts are also discussed in the context of co-catalysts. In the third part, we discuss examples of promising hybrid nanocatalysts, consisting of a molecular catalyst ML attached on the surface of a nanoparticle. In the same context, we discuss nanoparticulate co-catalysts and hybrid co-catalysts, consisting of catalyst attached on the surface of a nanoparticle, and their role in the performance of molecular catalysts ML.

Double-Nozzle Flame Spray Pyrolysis as a Potent Technology to Engineer Noble Metal-TiO2 Nanophotocatalysts for Efficient H2 Production

“…(c) Noble metals, particularly Pt 0 , can serve as reaction sites, i.e. due to low activation energy for surface reduction of H + towards H 2 [10,11].…”

A μ4‐Oxo Bridged Tetranuclear Zinc Complex as an Efficient Multitask Catalyst for CO2 Conversion

“…Though it is not necessarily the fact that all the metal centers do participate in the catalytic activity, which is not feasible due to steric factors but all are interconnected and must be interacting with themselves. For a more accurate comparison the TON and TOF values were calculated [19] w.r.t. the complex as well as per reactive site.…”

Progress in Catalytic Hydrogen Production from Formic Acid over Supported Metal Complexes

“…The reaction mixture except of formic acid contained sodium formate (9:1). It is known that addition of sodium formate to formic acid should give a higher activity [19,29,30]. The results showed that this resin ionically interacted strongly with the Ru complex and no Ru leaching took place during the reaction.…”

“…Novel single atom metal catalysts supported on N-doped carbon may provide a higher activity in the formic acid decomposition than the activity of the catalysts with nanoparticles, but the difference is not so significant [17]. The activity of homogeneous metal complexes is often higher [18][19][20][21][22][23][24]. Hence, they could be used at lower temperatures.…”